In any combustion reaction, it is desirable to ensure that the ratio of fuel to air is appropriate for achieving complete combustion of the fuel. Combustion in the presence of too much excess air tends to increase NOX production (and CO2 if there is too much excess air and too little water), while combustion with insufficient air may result in CO production or result in unburnt fuel in the form of soot and/or slag.
A problem, however, is the difficulty in being able to accurately monitor and control a combustion process. For example, changes in fuel and air composition during combustion can occur rapidly. The typical method of controlling combustion is by measurements on the exhaust gases which are ineffective since there can be a significant time delay between the moment of combustion and the time at which a need for correction is detected. Another problem is that there is no reliable way of determining the amount of combustible material (or potential energy content) in a given amount of fuel. This is a significant problem in the combustion of solid fuels (such as pulverised coal), which may contain different amounts of impurities in different samples of fuel being fed to a burner. As a result, it can be very difficult to work out the volume of air required for achieving complete combustion of a sample of fuel.
There is also a growing need for technologies that help reduce greenhouse gas emissions, and to use fuels such as coal more efficiently and effectively in energy production. Attempts have been made to minimise the release of pollutants (such as CO, CO2 and NOX) by improving the cleaning of flue gases after combustion. Attempts have also been made to develop more thermally efficient systems that use less coal to generate the same amount of power (e.g. using higher grade coal), together with improved techniques for effluent treatment and residue use and/or disposal. However, none of these approaches address the problem of detecting and correcting imbalances in fuel/air composition to minimise the production of undesirable gases.
There have been other attempts at monitoring and controlling combustion. WO 88/02891 describes a video image processing method for flame monitoring in a combustion process. Video cameras capture images of the flame from the side. The video signal is continually processed to find an ignition area of the flame (based on the gradient of pixel intensities). Temporal changes in the location of the ignition area are used to control the boiler. However, this technique focuses on a specific characteristic of the flame, and does not control combustion based on any physical characteristics of the inputs (e.g. fuel) used for combustion.
Another approach is described in WO 96/34233, which relates to a method of measuring the amount of pulverised fuel in a boiler for controlling a combustion process. Furnace cameras measure the distribution of heat radiation emitted by the flame over a predefined area. An irradiance value is determined for a point within the flame area at a set distance away from the ignition point of the flame. The air feed rate to the burner is determined simultaneously. The fuel feed rate is determined based on the irradiance value and the air feed rate (which enables the amount of pulverised fuel in the flame to be determined). The air feed rate is adjusted according to changes in the fuel feed rate. This technique does not control combustion based on any physical characteristics of the inputs used for combustion.
It is therefore desired to address one or more of the above problems, or to at least provide a useful alternative to existing combustion control techniques.